The construction of a transformer is prescribed by IEC (International Electrotechnical Communication) standards Pub. 950, 65, 335, 601, etc. Specifically, these standards provide 1) that an enamel film which coats a conductor of a winding be not authorized as an insulating layer, and that at least three insulating layers including an auxiliary insulating layer be formed between primary and secondary windings or 2) that the thickness of an insulating layer be 0.4 mm or more, for example, the creeping distance between the primary and secondary windings, which varies depending on the applied voltage, be 5 mm or more, and 3) that the transformer withstand a voltage of 3,000 V applied between the primary and secondary sides for a minute or longer, and the like.
Accordingly, in a currently prevailing transformer, a cross-section structure such as the one illustrated in FIG. 3 has heretofore been adopted. Specifically, flanged bobbin 2 is fitted in ferrite core 1, and enameled primary winding 4 is wound around bobbin 2 in the state such that insulating barriers 3 for securing the creeping distance are arranged individually at both ends of the periphery of bobbin 2. Then, insulating tape 5 is wound for at least three turns on primary winding 4, insulating barriers 3 for securing the creeping distance are arranged on insulating tape 5, and enameled secondary winding 6 is then wound around the insulating tape in a similar way.
In recent years, however, a transformer having a structure that includes neither insulating barrier 3 nor insulating tape layer 5, as shown in FIG. 2, has been beginning to appear instead of the transformer having the cross-section structure shown in FIG. 3. This transformer has advantages in that the overall size thereof can be reduced compared to the transformer having the structure shown in FIG. 3 and that winding work of insulating tape 5 can be omitted.
In the case of manufacturing the transformer shown in FIG. 2, it is required, in accordance with the IEC standards, that three insulating layers 4b, 4c and 4d, or 6b, 6c and 6d are formed on the outer periphery of one or both of conductors 4a or 6a of primary winding 4 and secondary winding 6 which are used. Further, the IEC standards require that, in the primary winding 4 and the secondary winding 6, each other's interlayers are identifiable among these insulating layers.
Such a winding is known to have a structure in which an insulating tape is wound on the outer periphery of a conductor to form a first insulating layer and then further the insulating tape is wound around thereon to form a second insulating layer and a third insulating layer in succession, so as to form a three-layer structure insulating layer in which interlayers, namely the number of insulating layers, are identifiable with respect to one another. Further, another winding is also known to have a structure in which a fluorine resin is extrusion-coated in succession on the outer periphery of a conductor enameled with polyurethane so that insulating layers are formed by an extrusion-coated layer having a three-layer structure as a whole (for example, see Patent Literature 1).
In addition, as an insulated wire having a multilayer insulating layer, for example, it has been proposed that in a multilayer insulated wire having a conductor and three or more extrusion-insulating layers coating the conductor, the multilayer insulated wire includes the innermost layer (B) of the insulating layers, the innermost layer (B) being composed of an extrusion-coated layer including resins which contain both a thermoplastic straight-chain polyester resin whose coefficient of extension, when the resin is soaked in a solder bath at 150° C. for 2 seconds, has a specific range and a resin containing an ethylene-based copolymer or a resin containing an epoxy group (Patent Literature 2).
Further, it has also been proposed that “in a multilayer insulated wire having two or more insulating layers on a conductor, the multilayer insulated wire has the insulating layers in which the outermost layer is composed of an extrusion-coated layer of a polyamide resin and the other layers are composed of extrusion-coated layers of polyether sulfone” (Patent Literature 3).